This invention is directed to a method and apparatus for raster image processing. More particularly, this invention is directed to a method and apparatus for raster image processing which uses page buffer management.
Typically, a raster image is used to output image data to an output device, such as a cathode ray tube monitor, an LCD, or a document output device, such as a printer. A raster image is represented as a series of scan lines of pixels, wherein each pixel is accessed in a sequential manner and each scan line is processed. An area to which image data may be output sequentially by an output device is referred to as a page.
In the past, there were sufficient memory resources in a typical output device or system to fully represent a single page in memory as a bitmap. A 600 by 600 dots per inch monochrome image would require approximately 4 megabytes of memory for an A4 page and approximately 8 megabytes of memory for an A3 page. However, as more users require color images, it is unlikely that current output devices or systems have adequate memory resources available to store a single page in memory. For example, a 600 by 600 dots per inch color image with 4 color planes that are each 8 bits deep, would require approximately 256 megabytes for an A3 page. If the resolution is increased to 1200 by 1200 dots per inch, the memory requirement is approximately 1 gigabyte. If the image data includes tag-bit data, the memory requirement for a single page is in excess of 300 megabytes for an A3 page.
Traditionally, raster image processes have been used to avoid such memory constraints by implementing display lists. Display lists allow raster image processing to operate in more constrained memory configurations and are more capable of handling memory constraints. Display lists also provide a generic representation of page data which is beneficial in supporting multiple page description languages, as well as a number of post raster image processing operations.
However, there are several problems with using display lists to avoid memory constraints. For example, even using display lists, there are some pages that cannot be accommodated or processed in some memory configurations. In addition, some raster image processing methods will not process image data having such large memory requirements. Further, using display lists requires a two pass raster image processing operation which requires additional overhead for display list maintenance and in processing time.
The subject invention overcomes the above-noted limitations and provides a method and apparatus for raster image processing which uses page buffer management.